The LTE (Long Term Evolution) cellular system specified by the 3GPP (Third Generation Partnership Project) provides a variety of location based services. These services all utilize the location of a UE (user equipment) e.g. mobile terminal for one purpose or the other. Currently one such important service is the E-911 emergency positioning functionality that is regulated for the US market. So-called E-911 phase 2 positioning requirements specify that all cellular networks have to be able to position users within 30 s, with accuracies better than 150 m (95%) and 50 m (67%), as counted for each county and each emergency center (PSAP). Considering the fact that GPS (Global Positioning System) receivers have very limited coverage indoors and the fact that most cell phone calls are placed indoors, these are very difficult requirements. The consequence is that the LTE cellular system standardizes not only GPS, but actually assisted Global Navigation Satellite System (A-GNSS) which is an enhancement of several of the coming and existing satellite navigation systems (of which GPS is one example). On top of that, a number of alternative positioning methods that rely on cellular network measurements are standardized in LTE, the details of these positioning methods are described below.
Emergency positioning requirements are also under way in other regions, like e.g. India; however, in markets outside the US, cell phone positioning is primarily used for commercial location based services such as personal and vehicular navigation, friend finding and geographical search services. Lawful surveillance and intercept are other situations where location technology is particularly useful.
To support the positioning methods the entire cellular infrastructure is prepared for processing and signaling of geographical position information. In LTE most of the positioning related functionality resides in the so-called eSMLC (evolved-Serving Mobile Location Center) node.
In areas with relatively low expected density of LTE user equipment, the uplink UL coverage, rather than the DL capacity, is typically the limiting factor. The base station node and in particular the high power parts thereof are expensive pars of the communication access network and in order to save costs a so-called Psi-coverage configuration can be used. In such a configuration, a single eNodeB is connected to three standard cross-polarized sector antennas. The downlink (DL) signals are divided onto the three antennas. The connection is made via two specially designed 3-way splitter-combiners. In this way, a maintained DL coverage can be obtained but with one eNodeB instead of three. However, to maintain the performance, the uplink UL signals are received from multiple antenna branches and combined into one radio unit, using different frequencies for different sectors, thereby reducing interference and noise. The interference between the combined signals originating from different antennas can therefore be reduced. This results in an UL sensitivity comparable to an ordinary 3-sector configuration. Compared to an omni configuration the UL capacity is substantially improved.
Since Psi-coverage maps the UL and DL into one omni sector-cell, the radio network controller views the configuration as a high capacity and high coverage omni-sector cell. In other words, the Psi-coverage approach involves an omni base station with three antennas, stripped of some but not all three-sector functionality. It is intended to have a very low cost and good coverage. This allows the normal functioning of most RAN (Radio Access Network) features. However, the positioning capabilities are limited, as only one cell area per omni sector-cell is defined for the positioning. In other words, the sector information is lost for positioning. Since a plurality of positioning methods in LTE rely on sector information to maintain accuracy, this has a large negative effect on the positioning accuracy. The same is valid for the closely related so-called Y-coverage wherein one eNodeB is connected to two sector antennas.
Therefore, there is a need for methods of improving the accuracy of positioning for Psi-coverage or Y-coverage configurations.